A strip line circuit generally consists of three dielectric layers including a central layer having conductor strips on either side, and two outer layers having a ground plane on the outside of each such layer. Considerable problems are encountered when an attempt is made to fold a strip line circuit assembly so as to achieve a space savings or to provide a package that can be used with other circuit elements that require the strip line to be in the folded condition. Such applications, requiring the bending of a flexible circuit assembly, include but are not limited to strip line antennas with dual coupled circuits which are laid out on the same multi-layer, flexible circuit board. A considerable amount of trouble has been experienced in fabricating such an antenna. The favored approach to date has been to etch the circuitry on a single multi-layer circuit board and to fold it along with the ground planes, back on itself through a curved one hundred-eighty degree bend to form two layers. To ease the problems associated with folding the strip line sandwich outer boards, which are normally thicker then the center board, three boards having a total thickness the same as one of the outer boards has been substituted for each of the outer boards. Thus, the total strip line sandwich is composed of seven boards. This has proven to be complex to assemble and performance, in general, has been found to be degraded, particularly where such performance is a function of the thickness of each of the layers of the board.
It is thus an object of the present invention to provide a multi-layer, flexible, and continuous strip line circuit assembly that can be folded back upon itself through a curved bend of one hundred-eighty degrees.
It is yet another object of the present invention to provide a reliable connection coupling two stacked strip line circuits.
It is further an object of the present invention to provide a method of fabricating the stacked strip line circuit assembly from a flexible, multi-layer, continuous strip line circuit by coupling through a one-hundred-eighty degree bend of the center layer bearing the circuit conductors.
Whether the need for a stacked, coupled, strip line circuit derives from an electronic applications requirement or a functionally related or unrelated packaging requirement, the present invention has been found to be most satisfactory as a solution. Essentially the invention consists of two stacked, coplanar strip line circuits coupled physically and functionally by a 180.degree. fold of only the center dielectric layer and the conductor leads on either side of that layer. The two other dielectric layers and the ground planes of each of the strip line circuits on the central dielectric layer are terminated, allowing the central dielectric layer to be bent around a curved support. The two strip line circuits are fabricated as one continuous planar central dielectric circuit upon which the two outer dielectric layers are impressed except for leaving open the central dielectric layer which is to be folded. Thus only a single layer of a multiple layer dielectric assembly needs to be folded to couple two stacked strip line circuits. The resulting circuit assembly is more reliable then previous folded circuits since there are no problems in terms of cracked dielectric boards, circuit traces, or ground planes. Further, performance is not degraded and, in effect, in many cases is improved, because there is no need to increase the thickness of the outer dielectric layers or the sandwiching of such layers.
In the specific case of antenna applications, the present invention has been found easy to assemble to an orthogonal balun connection to a spiral antenna.
These and other objects, features, and advantages of the present invention will be better understood with reference to the following discussion and the drawings which accompany it.